Measure tightly around your ribcage, just below your breasts.
Measure around the fullest part of your chest while wearing a non-padded bra.
Your Calculated Size:
How to Determine Your Bra Size Correctly
Wearing the correct bra size is crucial for comfort, posture, and breast health. Over 80% of women wear the wrong bra size, often settling for a band that is too large and cups that are too small. This calculator uses the modern "true fit" method to provide a starting point for your ideal size.
The Step-by-Step Measurement Guide
To get the most accurate results from the bra size calculator, follow these steps with a soft measuring tape:
Step 1: Underbust (Band Size) – Wrap the tape measure tightly around your ribcage directly under your bust. Make sure the tape is level and snug. If you get a fraction, round to the nearest whole number.
Step 2: Bust (Cup Size) – Wrap the tape loosely around the fullest part of your chest (usually across the nipple). Keep the tape level but do not pull it tight.
Understanding the Math
The calculation follows a two-part logic system:
Band Calculation: If your underbust measurement is an even number, that is usually your band size. If it is an odd number, we generally round up to the next even number for comfort.
Cup Calculation: We subtract the band size from the bust measurement. Each inch of difference represents one cup size. For example, a 1-inch difference is an A cup, and a 3-inch difference is a C cup.
Example Calculation
If your measurements are:
Underbust: 31 inches (Rounded to 32 Band)
Bust: 35 inches
Calculation: 35 (Bust) – 32 (Band) = 3 inches.
Result: 32C
What are "Sister Sizes"?
If a bra fits perfectly in the cup but feels tight in the band, or vice versa, you might need a sister size. A sister size maintains the same cup volume while changing the band. If you go up one band size, you must go down one cup letter (e.g., 34C to 36B).
function calculateBraSize() {
var underbust = parseFloat(document.getElementById('underbust').value);
var bust = parseFloat(document.getElementById('bust').value);
var unit = document.getElementById('unitType').value;
var resultArea = document.getElementById('resultArea');
var sizeResult = document.getElementById('sizeResult');
var sisterSizes = document.getElementById('sisterSizes');
if (isNaN(underbust) || isNaN(bust) || underbust <= 0 || bust <= 0) {
alert("Please enter valid positive measurements.");
return;
}
// Convert cm to inches for the standard calculation logic
var uInches = underbust;
var bInches = bust;
if (unit === 'cm') {
uInches = underbust / 2.54;
bInches = bust / 2.54;
}
// Calculate Band Size
// Standard logic: Round underbust to nearest even number
var band = Math.round(uInches);
if (band % 2 !== 0) {
band += 1;
}
// Calculate Cup Difference
var diff = Math.round(bInches – band);
var cups = ["AA", "A", "B", "C", "D", "DD/E", "DDD/F", "G", "H", "I", "J", "K", "L", "M", "N"];
var cupLabel = "";
if (diff = cups.length) {
cupLabel = "Larger than N";
} else {
cupLabel = cups[diff];
}
// Prepare sister sizes
var sisterUpBand = (band + 2);
var sisterUpCup = (diff – 1 >= 0) ? cups[diff – 1] : "N/A";
var sisterDownBand = (band – 2);
var sisterDownCup = (diff + 1 < cups.length) ? cups[diff + 1] : "N/A";
var finalSize = band + cupLabel;
sizeResult.innerHTML = finalSize;
sisterSizes.innerHTML = "Sister Sizes: Try " + sisterUpBand + sisterUpCup + " for a looser band, or " + sisterDownBand + sisterDownCup + " for a tighter band.";
resultArea.style.display = 'block';
resultArea.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}